This invention relates to a power semiconductor switch, in particular a power semiconductor switch comprising a power semiconductor device having a first main electrode for coupling to a first voltage supply line, a second main electrode coupled to a first terminal for connection via a load to a second voltage supply line and an insulated gate electrode coupled to a control terminal for supplying a gate control signal to enable conduction of the power semiconductor device and an arrangement for determining whether or not the load is present, that is for determining whether or not the load is open-circuited. Such an open-circuit detection arrangement may be used to determine whether, for example, a lamp or bulb operated by the power semiconductor switch has blown. The invention also relates to an automotive electrical system comprising such a power semiconductor switch.
Where a power semiconductor device such as an n-channel enhancement mode power MOSFET is being operated as a low-side switch, that is where the voltage supplied to the second voltage supply line is more positive than the voltage supplied to the first voltage supply line, then the voltage between the first and second main electrodes of the power semiconductor device will depend on the status of the power semiconductor device and of the load.
Thus, when the power semiconductor device is switched off (that is it is not conducting) by removing the gate control signal and the load is connected (that is the load is not open-circuited), then the voltage at the second main electrode of the power semiconductor device will be high because it will be at the potential of the voltage supply, generally battery voltage where the protected switch is being used for an automotive application. If, however, the load becomes open-circuited for some reason, then the voltage between the first and second main electrodes of the non-conducting power semiconductor device will be zero.
In contrast, when the power semiconductor device is conducting, then the voltage between the first and second main electrodes will be very low when the load is operating normally (because the on-resistance of the power semiconductor device will be low compared to the resistance of the load) and will be zero when the load is open-circuited. It is thus very difficult to detect when a load is open-circuited when the power semiconductor device is conducting.
It ought to be relatively easy to check whether a load is operating normally (ie is not open-circuited) when the power semiconductor switch is not conducting, that is when the gate control signal is not applied to the insulated gate electrode of the power semiconductor device. However, even if the load itself is faulty, moisture or dirt in the applications environment may induce a leakage current that may pull the voltage at the second main electrode of the power semiconductor device high , even though the load is open-circuited. Accordingly, the presence of such a leakage current path may make it difficult to detect an open-circuited load even when the power semiconductor device is not conducting.
Previous attempts at detecting an open-circuit condition of a load have involved the user of the power semiconductor switch, for example an automobile manufacturer, coupling an external resistor between the first and second main electrodes of the power semiconductor device to draw off a portion of the current through the power semiconductor device in an attempt to detect whether the load is open-circuited when the power semiconductor device is non-conducting.
The use of such an external resistor means, however, that there is a permanent leakage of current from the power supply that may in time cause a drain on the power supply and could result in a flat battery where the power semiconductor switch is being used for an automotive application. Moreover, there is a danger of the external resistor becoming detached so that the power semiconductor switch is no longer able to indicate whether the load is open-circuited. In addition, such a technique involves additional expense and effort for the user of the power semiconductor switch.